1. Field of the Invention
The present invention relates to measuring devices, in preferred embodiments to electromagnetic flow meters or Coriolis mass flow meters. As used herein, the term “flow meter” is intended to encompass flow meters for measuring flow of a liquid.
2. Description of Related Art
Electromagnetic flow meters are well known in the art. Very briefly, in an electromagnetic flow meter one or more excitation coil(s) are arranged in or around a measurement duct for generating a magnetic field across the fluid flowing through the measurement duct. This causes electric charges within the fluid to be displaced transversely to the direction of the magnetic field and the direction of flow, which results in a potential difference between opposite sides of the measurement duct. This potential difference can be measured and provides an indication of the flow through the measurement duct.
In an AC electromagnetic flow meter (i.e. an electromagnetic flow meter whose coils are energised by an AC current) the measured signal will also generally be an AC signal.
Coriolis mass flow meters are known for example from U.S. Pat. Nos. 4,422,338, 5,423,221, 4,856,346, 5,394,758, 4,192,184 and U.S. re-issue Pat. No. 31450, the disclosures of each of which are herein incorporated by reference.
Briefly, in an AC Coriolis mass flow meter a measurement duct is caused to oscillate, for example by twisting the duct by means of AC electromagnetic excitation. This displacement of the measurement duct at various positions along the duct is indicative of mass flow through the measurement duct. The measured signal indicative of displacement will also generally be of AC nature.
In an AC Coriolis mass flow meter it is important to know the phase between the AC excitation and the measured signal since a measure of mass flow can be derived from the phase. In an electromagnetic flow meter phase sensitive detectors can be used to improve accuracy.
There are other instances in measuring devices where an accurate phase measurement or phase sensitive detection may be required.
In a conventional approach the phase is determined, using a zero-crossing technique. Another conventional approach employs peak detection. Phase locked loops may also be used.
Modern measuring devices often include a digital signal processor (DSP) for digitally generating an excitation signal, digitally filtering a measured signal and this can be used to carry out digital zero-crossing detection and peak detection.